Problem with SF800 flow sensor reading

Question

I am having a problem with my flowsensor circuit as show in the figure.

where SF800 is the flow sensor I am using. It's a pulse sensor PNP/NPN contact. Hence the pulse signal pin in pull up using a 2.2 kΩ resistor as the manufacturer says. The K - Factor of the sensor given by the manufacturer is around 5600 pulses per litre. SF800 sensor is provided with a 3/8-inch NPT male threaded connection.

// Interrupt handler
static void IRAM_ATTR flow_sensor_isr_handler(void* arg) {
  if (PULSE_TASK_FLAG) {
    PULSE_COUNTER++;
  }
}


void init_flow_sensor() {
    // Configure GPIO
    gpio_config_t io_conf = {
        .pin_bit_mask = (1ULL << GPIO_PULSE_SENSOR_IO),
        .mode = GPIO_MODE_INPUT,
        .pull_up_en = GPIO_PULLUP_ENABLE,
        .pull_down_en = GPIO_PULLDOWN_DISABLE,
        .intr_type = GPIO_INTR_NEGEDGE  // Falling edge trigger
    };
    gpio_config(&io_conf);

    // Install ISR service
    gpio_install_isr_service(ESP_INTR_FLAG_NMI);

    // Attach interrupt handler
    gpio_isr_handler_add(GPIO_PULSE_SENSOR_IO, flow_sensor_isr_handler, NULL);
}


static void pulse_task (void *arg) {
  const char * tag = "PULSE Task ";
  char packet[64];

  // Initialize flow sensor
  init_flow_sensor();
  // Initialize last time
  lastTime = esp_timer_get_time();

  while (1) {
    // ESP_LOGI(tag, "Pulse Test");
    if (PULSE_TASK_FLAG) {
      int64_t currentTime = esp_timer_get_time();
      int64_t deltaTime = currentTime - lastTime;

      if (deltaTime >= 100000) {  // Update every 1 second (in microseconds)
        uint32_t currentPULSE_COUNTER = PULSE_COUNTER;
        PULSE_COUNTER = 0;  // Reset counter

        // Calculate flow rate and total flow
        FLOW_RATE = (currentPULSE_COUNTER / (float) K_FACTOR) * 600;  // Convert pulses/sec to LPM
        TOTAL_FLOW += FLOW_RATE * (deltaTime / 60000000.0); // Add to total flow (convert us to minutes)

        // ESP_LOGI(TAG, "Delta: %lld, Flow Rate: %.2f LPM | Total Flow: %.2f L", deltaTime, FLOW_RATE, TOTAL_FLOW);

        lastTime = currentTime;
      }
      // Send FUEL Data to the HMI
      gen_packet("FUEL", "PUL", TOTAL_FLOW, packet, sizeof(packet));  // Generate packet
      sendData(tag, packet);
      vTaskDelay(pdMS_TO_TICKS(10));

      if (DEF_SET_VALUE && TOTAL_FLOW >= DEF_SET_VALUE) {
        pump_stop();
        ESP_LOGI(tag, "Set value %.2f reached! --- %.2f", DEF_SET_VALUE, TOTAL_FLOW);
      }
    } else {
      // Pause counting when the flag is not set
      //
    }
    vTaskDelay(pdMS_TO_TICKS(10));
  }
}

The problem I am facing is that when I test it for 5 litres I am getting an error of (+/-)30 ml. (Say for 5 litres I get from 4.98 to 5.10 litres. Also when I go for 10 litres the error buildup is also increasing (but no linearly). Also it varies if I decrease the flow through the sensor using a valve). Hence this error is not at all acceptable. I need the error to be minimal say 20 ml max.

I am getting a doubt that there should be some mistake in my code. SF800 is said to be an accurate sensor [No Detailed Datasheet is available for the sensor other than its connection diagram]. The code is done based on ESP IDF framework. The code block containing the pulse task is attached above. The Pulse task is a FreeRTOS Task which run independantly.

The PULSE_TASK_FLAG is set high when the motor is switched ON, which causes the water to flow through the sensor. Then the task starts calculating the TOTAL_FLOW through the sensor until the motor is Switched OFF. flow_sensor_isr_handler is an interrupt handler which counts the pulses.

I would be grateful for any recommendations of alternative sensors that you have successfully implemented in previous projects.

If there is a requirement for any more information I can provide it.

Answer 1

When I test it for 5 litres I am getting an error of ±30 ml.

1% of 5,000 ml = ±50 ml.

At ±30 ml you're doing well.

I need the error to be minimal say 20 ml max.

It looks like it is not possible with this sensor.